Improved thermal contact between cryogenic refrigerators and cooled components

ABSTRACT

An arrangement for mounting a two stage cryogenic refrigerator ( 17 ) into a cryostat, the arrangement comprising a vacuum sock ( 15 ) for accommodating at least a part of the refrigerator, attachment means ( 32, 34 ) for attaching an upper part of the refrigerator to a surface ( 14 ) of the cryostat around an opening of the vacuum sock, a thermally conductive portion ( 26 ) of a wall of the vacuum sock which, in use, is thermally and mechanically in contact with a second cooling stage ( 24 ) of the refrigerator, and arrangements ( 40, 42 ) are provided for thermally connecting a first stage ( 22 ) of the refrigerator to a thermal radiation shield ( 16 ) of the cryostat.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improved arrangements for providingthermal connection between a cryogenic refrigerator and cooledcomponents, wherein the refrigerator is removable, and the thermalconnection must be capable of being broken and re-made withoutdiscernable increase in thermal resistance.

2. Description of the Prior Art

The present invention is described in the context of a cryogenicrefrigerator cooling to temperatures of about 4.2K for re-condensinghelium in a cryostat used for cooling superconducting magnets for MRIsystems.

FIG. 1 shows a conventional arrangement of a cryostat including acryogen vessel 12. A cooled superconducting magnet 10 is provided withincryogen vessel 12, itself retained within an outer vacuum chamber (OVC)14. One or more thermal radiation shields 16 are provided in the vacuumspace between the cryogen vessel 12 and the outer vacuum chamber 14. Insome known arrangements, a refrigerator 17 is mounted in a refrigeratorsock 15 located in a turret 18 provided for the purpose, toward the sideof the cryostat. Alternatively, a refrigerator 17 may be located withinaccess turret 19, which retains access neck (vent tube) 20 mounted atthe top of the cryostat. The refrigerator 17 provides activerefrigeration to cool cryogen gas within the cryogen vessel 12, in somearrangements by recondensing it into a liquid. The refrigerator 17 mayalso serve to cool the radiation shield 16. As illustrated in FIG. 1,the refrigerator 17 may be a two-stage refrigerator. A first coolingstage 22 is thermally linked to the radiation shield 16, and providescooling to a first temperature, typically in the region of 80-100K. Asecond cooling stage 24 provides cooling of the cryogen gas to a muchlower temperature, typically in the region of 4-10K.

A negative electrical connection 21 a is usually provided to the magnet10 through the body of the cryostat. A positive electrical connection 21is usually provided by a conductor passing through the vent tube 20.

Typically, the cryogenic refrigerator will be a two-stage refrigerator,providing high-power cooling to a first cryogenic temperature andlower-power cooling to a much lower cryogenic temperature, asillustrated in FIG. 1. In current cryogenic refrigerators, the firststage may provide about 44 W of cooling to 50K and about 1 W of coolingat about 4K. Typically, a first stage heat exchanger 22 is in thermalcontact with the thermal radiation shield 16 as illustrated in FIG. 1.

In some conventional systems, a second stage heat exchanger is exposedto a gaseous cryogen environment in the present example, gaseouscryogen. The second stage is cooled to a temperature below the boilingpoint of the cryogen, which condenses onto the second stage heatexchanger. Such arrangements provide direct contact between cryogen andsecond stage heat exchanger, but care must be taken when removing andreplacing the refrigerator, since air will tend to be drawn into thecryogen vessel, where it will freeze onto surfaces, and may causedangerous blockages. The service operation of removing and replacing therefrigerator with the magnet at field is also a hazardous operation, asa quench could take place while the refrigerator is absent, placing aservice technician at risk from exposure to liquid and gaseous cryogen.

FIG. 2 schematically illustrates a conventional arrangement in which thecryogenic refrigerator is housed within an enclosure 15, colloquiallyknown as a “vacuum sock”, sealed from the interior of the cryogen vessel12. In this case, the second stage heat exchanger 24 is in thermalcontact with gaseous cryogen in the cryogen vessel 12 through a part 26of a wall of the vacuum sock 15. A heat exchanger surface 28 may beprovided on the cryogen vessel side of this part 26 of the wall, toenhance thermal transfer, for example having a finned and/or texturedsurface. Cooling in this way, by conduction through a wall of the vacuumsock, introduces thermal resistance between the second stage 24 of therefrigerator and the cryogen gas, but provides the advantage that thecryogenic refrigerator 17 may be removed and replaced without exposingthe interior of the cryogen vessel 12 to air. Air may enter the vacuumsock 15, but this will solidify inside the vacuum sock when therefrigerator is in use, and does not pose a risk of dangerous blockage.The thermal connection between the first cooling stage 22 and thethermal radiation shield 16 may be provided by a tapered cooling stage22 and a tapered interface block 30.

It is of course important to ensure effective thermal transfer betweenthe first cooling stage 22 of the refrigerator 17 and the thermalradiation shield 16. This may be achieved, as illustrated, using atapered first cooling stage 22 and a tapered interface block 30 which isthermally and mechanically joined to the thermal radiation shield 16.The first cooling stage 22 and the interface block 30 are each typicallyof copper, and the taper angle a is chosen to be narrow enough to ensurea high enough pressure between the surfaces of the first cooling stage22 and the interface block 30 to ensure good thermal conductivity, butnot so narrow an angle that the refrigerator 17 becomes difficult toremove. At an upper end of the refrigerator 17, a flange 32 is bolted 34to the surrounding surface of the cryostat OVC 14. The dimensions of thevarious components are carefully calculated such that the first coolingstage 22 and the interface block 30 are driven together with anappropriate force as the refrigerator is tightened into position bybolts 34. Some flexibility in the mounting of the interface block 30restricts the maximum force to an appropriate level, and allows for sometolerance in the respective dimensions.

Thermal connection must also be provided from the second cooling stage24 through the wall of the vacuum sock 15. Typically, a part 26 of thewall which contacts the second stage 24 will be of a thermallyconductive material such as copper, and may be profiled to provide anenhanced surface 28 for heat exchange on the side which is exposed tothe interior of the cryogen vessel. For example, that surface may befinned and/or textured. In certain known arrangements, the variouscomponents are dimensioned such that the second cooling stage 24 pressesin to wall part 26 with appropriate force and the tapered first coolingstage 22 meets the tapered interface block 30 with appropriate force asthe flange 32 is bolted 34 on to the surrounding surface of the cryostatOVC 14. Conventionally, a compliant interface material, typically anindium washer, may be placed between mating surfaces of the wall part 26and the second cooling stage 24 to allow effective thermal connectionwhile allowing some tolerance in mechanical position. A difficulty withsuch an arrangement is that the indium washer is destroyed when therefrigerator is removed, and it is difficult to remove all traces of anold indium washer from the inside of the vacuum sock 15. Any remainingtraces of an old indium washer will degrade the thermal interfaceprovided by a replacement indium washer.

In the prior art arrangements as discussed above, efficient thermalinterfaces between the refrigerator and cooled components have reliedupon precise mechanical dimensions. Mechanical force applied whenbolting 34 the flange 32 of the refrigerator 17 in place is sharedbetween sealing the refrigerator to the surrounding surface of thecryostat OVC 14, and interface forces between the first cooling stage 22and the interface block 30; in some embodiments, also interface forcesbetween the second cooling stage 24 and the part 26 of the wall of thevacuum sock. This sharing of forces means that any mechanical tolerancein respective dimensions will change the proportions of force applied ateach interface, resulting in unpredictable thermal resistances of thevarious interfaces. This is usually overcome at the first stage byadding additional thermal links with braids and an axial springmechanism to allow for build tolerances at the expense of less efficientthermal transfer, caused by an increased number of thermal joints.

SUMMARY OF THE INVENTION

An object of the present invention is to address these problems byproviding mounting arrangements for a cryogenic refrigerator wherein aninterface force is applied to the first cooling stage 22 in a directionperpendicular to a direction of insertion of the refrigerator 17. Theinterface force applied to the first cooling stage 22 is therebyindependent of the mechanical arrangements for mounting therefrigerator. Accordingly, the mounting force and the interface forcemay be independently optimized for their respective functions. When thesecond cooling stage 24 is also subjected to an interface force, theinterface force applied to the first thermal interface is in a directionperpendicular to an interface force applied to the second cooling stage.The first stage thermal interface force is independent from the secondstage thermal interface force. Increasing one thermal interface forcewill have no effect on the other thermal interface force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section through a conventional cryogenically cooledmagnet for MRI imaging.

FIG. 2 shows a conventional mounting arrangement for a refrigerator in acryostat.

FIG. 3 shows a mounting arrangement for a refrigerator in a cryostat,according to an embodiment of the present invention.

FIG. 4 shows a detail of another mounting arrangement for a refrigeratorin a cryostat, according to an embodiment of the present invention.

FIG. 5 shows an axial view of certain features of another embodiment ofthe present invention.

FIG. 6 shows an axial view of certain features of another embodiment ofthe present invention.

FIG. 7 shows a detail of a mounting arrangement for a refrigerator in acryostat, according to an embodiment of the present invention.

FIG. 8 shows a mounting arrangement for a refrigerator in a cryostat,according to an embodiment of the present invention.

FIG. 9 illustrates a detail of a bellows arrangement used in anembodiment of the present invention.

FIG. 10 illustrates a detail of an alternative bellows arrangement usedin another embodiment of the present invention.

FIG. 11 illustrates an optional feature which may be employed withbellows arrangements according to certain embodiments of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to an aspect of the present invention, a thermal link betweenthe thermal radiation shield and the first cooling stage 22 of thecryogenic refrigerator 17 is retracted when a refrigerator is insertedor removed, and the thermal link is pressed into contact with the firstcooling stage 22 when in operation by a force acting perpendicular to aforce applied to the refrigerator to seal it to the cryostat.

FIG. 3 shows a generic illustration of a first embodiment of the presentinvention. A vacuum sock 15 is provided, and second cooling stage 24 isin thermal and mechanical contact with a part 26 of the wall of the sock15. In use, the refrigerator 17 is inserted into the sock 15, and issealed and mounted in place to the surrounding surface of the cryostatOVC 14. The interior volume of the vacuum sock 15 may be evacuatedthrough a valve 36 or may effectively be evacuated by cryo-pumping whenoperational: as the refrigerator cools, any air components in the vacuumsock will freeze onto the second cooling stage 24. The vacuum load onthe refrigerator: that is, atmospheric pressure acting on the exposedsurfaces of the refrigerator, acts to make a firm joint between thesecond cooling stage 24 and the part 26 of the wall of the vacuum sock15. None of that load is used to make a thermal contact between thefirst cooling stage 22 and the thermal radiation shield 16. As arelatively high force is available for making the joint between thesecond cooling stage 24 and the part 26 of the wall of the vacuum sock15, it has been found that an effective thermal joint may be madewithout the use of an indium washer or similar between the secondcooling stage 24 and the part 26 of the wall. The entire vacuum load onthe top of the refrigerator is available to make the joint between thesecond cooling stage 24 and the part 26 of the wall. The refrigeratormay be bolted 34 or similarly attached to the surrounding surface of thecryostat OVC 14. The force applied to the refrigerator by the bolts 34or similar fasteners will add to the vacuum load and may contribute tothe pressure at the contact surface between the second cooling stage 24and the part 26 of the wall of the vacuum sock 15. If the vacuum force,and the force applied by bolts, is too high for the second thermalinterface, it may be reduced by adding springs under the flange 32, forexample around the bolts 34.

According to a feature of this embodiment of the invention, bellows 40are provided, containing an amount of cryogen which remains gaseous atthe temperature of operation of the first cooling stage 22. The cryogenmay be helium. The bellows 40 themselves are of thermally conductivematerial, such as copper or brass, and carry a contact piece 42 ofcopper or brass at an extremity nearest the refrigerator 17. At theother extremity, the bellows is thermally linked to the thermalradiation shield 16, either by thermal conduction through a thermalplate 43 attached to a wall of the vacuum sock (as illustrated), orthrough an aperture in the wall of the vacuum sock, the aperture beingclosed by the bellows and/or an arrangement thermally linking thebellows to the thermal radiation shield. Preferably, a plurality of setsof bellows is provided, equally spaced around the first cooling stage22. The interface block 30 of FIG. 2 may not be required, representing avaluable saving in cost and weight.

When the vacuum sock 15 is opened to atmosphere, the bellows 40 aredriven into a retracted position by atmospheric pressure, asillustrated. The refrigerator 17 may be installed or removed withoutinterfering with the bellows 40. When the vacuum sock is evacuated,either by pumping out through a valve 36 or by cryo-pumping by therefrigerator in use, the pressure within the vacuum sock 15 will fall,and the pressure of the cryogen gas within the bellows 40 will cause thebellows to expand, pressing the contact piece 42 into contact with thefirst stage 22 of the refrigerator. A thermally conductive path isaccordingly established between the thermal radiation shield 16 and thefirst cooling stage 22. The pressure applied by the bellows 4 0 onto thefirst cooling stage 22 is determined by the characteristics of thebellows and the quantity and nature of cryogen gas sealed into thebellows. The vacuum load acting on the top of the refrigerator plays nopart in determining the pressure between the contact piece 42 and thefirst cooling stage 22. As the first cooling stage thermal interfacepressure is independent from the second cooling stage thermal interfacepressure, both are easy to control. This is advantageous becausecontrolled thermal interface pressure enables accurate calculation andprovision of effective thermal contact which is derived from pressureand surface area.

FIG. 4 shows a detailed view of an alternative bellows arrangement.Here, the contact piece 42 is connected by a flexible thermallyconductive joint 44, such as an aluminum or copper braid or laminate, toa block 46, thermally linked to the thermal radiation shield 16. In suchembodiments, no thermal conduction need take place through the materialof the bellows 40. The bellows may then be of any material of suitablemechanical properties, without being constrained to materials of highthermal conductivity.

If required, mating surfaces of the contact piece 42 and the firstcooling stage 22 may have a thin coating of thermally conductive greaseor an indium contact to reduce thermal resistance between the twopieces.

FIG. 5 shows an example embodiment, which may be an embodiment as shownin FIG. 3, or an embodiment as shown in FIG. 4, when viewed in directionV.

As shown, the first stage 22 of the refrigerator is circular in plan, asis conventional. The contact pieces 42 are provided with a correspondingconcave surface 50 to increase a contact surface area with the firststage 22. As shown, multiple contact pieces and corresponding multiplebellows may be provided to increase the contact area with the firststage, and reduce the thermal resistance between the first stage 22 andthe thermal radiation shield 16 by providing multiple thermal paths inparallel. At 52 are represented through-holes, into which thermallyconductive braids may be attached, for example by soldering, inembodiments such as shown in FIG. 4.

FIG. 6 shows a similar view to that shown in FIG. 5, of an alternativeembodiment. Here, instead of using contact pieces which are shaped tomatch the surface of the first cooling stage 22, thermally conductiveblocks 54, for example of copper or aluminum are attached to the firstcooling stage, for example by bolting or similar. The thermallyconductive blocks 54 each provide a flat mating surface 56 for pressedcontact with a corresponding contact piece 42 carried by a bellows 40.Such an arrangement may be found easier to manufacture than the profiledcontact blocks shown in FIG. 5.

FIG. 7 illustrates another version of the present invention. Here,thermal contact between the first cooling stage 22 and the first stageinterface block 30, which is thermally connected to the thermalradiation shield 16, is provided by a thermal bus bar 58 which isprovided with a flexible part, here shown as a joggle 60 in the profileof the bus bar 58. As shown, the vacuum sock 15 is at atmosphericpressure, the bellows are retracted, and the bus bar 58 is not incontact with the first cooling stage. The refrigerator 17 may beinserted or removed without interfering with the bus bar 58, bellows 40or any of the thermal paths to the thermal radiation shield 16. In use,the vacuum sock 15 will be evacuated. The pressure of cryogen enclosedwithin the bellows 40 will cause the bellows to elongate. The bellows 40and contact piece 42 will bear upon the thermal bus bar 58 and bend itabout its flexible portion 60 such that it enters into thermal contactwith the first cooling stage 22. Such embodiments are advantageous inthat no modification needs to be made to the refrigerator 17 itself:there is complete freedom in choice of material of the bellows, as nothermal conduction need take place through the bellows. The materialcross-sectional area of the thermal path through the bus bar 58including its flexible part 60 may be significantly greater than thematerial cross-sectional area of the bellows, braiding or laminate 44used to conduct heat in the other embodiments discussed above.

FIG. 8 shows another embodiment of the present invention. Here, thefirst stage thermal intercept block 30 is a relatively close fit aroundthe first cooling stage 22 of the refrigerator 17. An upper surface 62of the first stage thermal interface block 30 is preferably tapered toassist installation of the refrigerator 17. When correctly aligned, thefirst cooling stage 22 passes unimpeded through the first stage thermalinterface block 30. As shown, one or more recesses or ports 64 areprovided in the material of the first thermal interface block 30. Abellows 40 is provided in each of the recesses or ports 64. Each of thebellows 40 may be arranged according to any of the embodiments describedabove, or any similar arrangement. In the embodiment schematicallyrepresented in FIG. 8, the bellows may correspond to the embodiment ofFIG. 3: the bellows are of a thermally conductive material and the firstcooling stage 22 cools the thermal radiation shield 16 by conductionthrough contact piece 42, bellows 40 and interface block 30 to theshield 16. As in the case with the other embodiments described, theinterface force of the thermal contact with the first cooling stage 22is directed radially to an elongate axis A-A of the refrigerator, andperpendicular to an interface force of the thermal contact with thesecond cooling stage, which is directed parallel to the elongate axisA-A of the refrigerator.

FIG. 9 shows a detailed representation of a bellows arrangement of aparticular embodiment of the invention. Here, a flexible thermalconductor 64 is provided, such as a copper or aluminum braid. One end ofthe flexible conductor 64 is affixed to an interface piece 42 providedat a radially inner end of the bellows 40. The flexible conductorextends the length of the bellows 40 to a fitting (not shown), inthermal contact with the thermal radiation shield 16. The bellows isnaturally extended in its “rest” state and is forced into a retractedposition when the vacuum sock is at atmospheric pressure.

FIG. 10 shows a detailed representation of another bellows arrangementof the present invention. This arrangement is somewhat similar to thearrangement of FIG. 9, in that a flexible thermal conductor 64 extendsthrough the bellows 40, and is joined to interface piece 42 at a surfacewithin the bellows 40. At the radially outer end of the bellows, asimilar interface piece 68 is provided, and sealed to a wall of thevacuum sock 15 with an end plate 66. Another thermal link (notillustrated) will be provided between the interface piece 68 and thethermal radiation shield 16. The force and pressure applied by thebellows at the first thermal interface may be varied by design of thebellows and the interface piece 42. Increasing the cross-sectional areaof the bellows will increase the interface force, as will increasing thelength of bellows in the radial direction. Reducing a surface area ofthe interface piece 42 will not change the interface force, but willraise the interface pressure.

FIG. 11 schematically represents an improved embodiment of the presentinvention. Here, a small bore pipe 70 is shown, communicating with aninterior volume of each of the bellows 40. Another end of the pipe 70passes through the wall of the cryostat to a supply of a gas. Ratherthan relying simply on a difference in pressure between the interior ofthe vacuum sock 15 and the interior of the bellows 40 which contains afixed mass of cryogen gas, this embodiment allows an increased pressureto be provided within the bellows 40 by adding a gas such as neon, argonor helium once the vacuum sock is at vacuum. This may allow improvedthermal conductivity between the interface piece 42 and the firstcooling stage 22 by increasing the contact pressure between theinterface piece 42 and the first cooling stage. Some heat transfer alsotakes place through the gas in the bellows. Preferably, gas is removedthrough pipe 70 from the bellows 40 when the refrigerator 17 is to beremoved, allowing the bellows to retract away from the first coolingstage, providing clearance for removal of the refrigerator.

FIGS. 12-16 represent a series of further embodiments, in which thermalinterface pieces are arranged to rotate about certain axes into contactwith refrigerator first stage 22 when the refrigerator is in positionand under vacuum in the sock 15, and to rotate out of contact with therefrigerator first stage 22 when the interior of the sock 15 is atatmospheric pressure during servicing operations. In some embodiments,one or more bellows is used, which contains a sealed mass of cryogengas, such that the bellows will elongate when the sock 15 is at vacuum,and will retract when the interior of the sock is at atmosphericpressure. In other embodiments, one or more bellows are provided with apipe 70, as described with reference to FIG. 11, which allows thepressure within the respective bellows to be controlled at will.

FIG. 12 schematically illustrates an axial view of a radially outersurface of the refrigerator first stage 22 with clamping contact pieces72 in position, in contact with the refrigerator first stage. Clampingcontact pieces 72 pivot about axle 74. In their closed position,illustrated, radially inner surfaces 76 of the clamping contact pieces72 are pressed into contact with a radially outer surface 78 of therefrigerator first stage 22. The radially inner surfaces 76 of theclamping contact pieces 72 are shaped to provide a large contact surfacearea between the clamping contact pieces and the refrigerator firststage 22. Part of a wall of sock 15 is schematically shown. According tothis embodiment of the invention, sealed bellows units 40 are provided,each between an actuator 80 attached to, or forming part of, eachclamping contact piece 72 and a bearing surface 82 mechanicallyrestrained in a fixed position with respect to the sock 15. Axle 74 ispreferably also restrained in position with respect to the sock 15 tocarry some of the weight of the clamping contact pieces 72. FIG. 12illustrates the assembly in the case that the sock 15 is evacuated. Apredetermined mass of a cryogen gas is sealed into each bellows 40. Whenthe sock is evacuated, the pressure of the cryogen within the bellowscauses it to elongate, and to bear against the respective actuator 80and bearing surface 82. The bellows accordingly presses the contactpieces 72 into thermal and mechanical contact with the first stage ofthe refrigerator. A thermal link, such as any of those described abovewith reference to other embodiments may be used to provide a coolingpath from the contact pieces 72 to the first stage of the sock, and soto the thermal radiation shield. The bellows may be adapted in length,and diameter to provide an appropriate force to clamp the contact pieces72 against the first stage of the refrigerator.

When the sock is at atmospheric pressure, the pressure differentialbetween the sock and the bellows will reduce and may even reverse insign. This will cause the bellows to compress. The ends of the bellows40 are respectively attached to the actuator 80 and the bearing surface82, and the contracting bellows disengage the surfaces 76 of the contactpieces 72 from the first stage 22 of the refrigerator. The contactpieces 72 are shaped in the region of the axle 74 to ensure that anuninterrupted clearance space is provided around the first stage of therefrigerator when the bellows are compressed: when the sock is atatmospheric pressure. This allows the refrigerator to be removed andreplaced unimpeded.

FIG. 16 shows a possible arrangement of the contact pieces 72 adjacentto the axle 74, in the direction XVI shown in FIG. 12. Ends of thecontact pieces interlock and axle 74 passes through both of them. Athermal connector 84 may conveniently be attached to the contact piecesat the axle 74.

In alternative embodiments, fewer or more than two contact pieces may beprovided, each associated with a bellows 40, and axle 74 and a bearingsurface 82. In other embodiments, the actuator 80 may be dispensed with,the bellows 40 arranged essentially radially to bear against a part ofeach contact piece preferably distant from the corresponding axle 74.FIGS. 12A, 12B and 12C schematically illustrate such embodiments.

FIG. 13 schematically illustrates another type of embodiment, whereinthe contact pieces are assembled in a manner similar to a pair ofpliers, in that a part which contacts the first stage 22 of therefrigerator is arranged on one side of an axle 74, while an extensionpiece 88 extends on the opposite side of the axle and is used to actuatethe arrangement. In the illustrated embodiment, each contact piece isactuated with a corresponding bellows 40. When the sock 15 is atatmospheric pressure, each bellows 40 is compressed, and the surface 76of each contact piece is pulled away from the surface 78 of the firststage of the refrigerator 22. A spring (not shown) may be provided topush the contact pieces away from the first stage 22 of therefrigerator. When the sock is at vacuum, the cryogen gas within eachbellows causes the bellows to elongate, and the contact pieces 72 topress into contact with the first stage 22 of the refrigerator. Otherfeatures shown in FIG. 13 correspond to features of FIG. 12.

In variants of the embodiment of FIG. 13, more or fewer than two contactpieces may be provided, each with their own axle. FIG. 13A illustratesone of these variants.

FIGS. 14, 15, 17 illustrate embodiments in which a pipe 70 is providedto introduce or remove cryogen gas from each bellows. Such anarrangement has been discussed above with reference to FIG. 11. In sucharrangements, cryogen gas is introduced into the bellows 40 when thesock is at atmospheric pressure and drives the contact pieces 72 out ofcontact with the first stage 2 of the refrigerator. When the sock is atvacuum, cryogen gas is withdrawn from the bellows, to pull the contactpieces 72 into contact with the first stage 22 of the refrigerator.Numerous variations of such embodiments are possible within the scope ofthe invention, as will be apparent to those skilled in the art in amanner similar to the variants shown in FIGS. 12-13A.

1. An arrangement for mounting a two stage cryogenic refrigerator,having first and second cooling stages in succession, into a cryostat,the arrangement comprising: a vacuum sock at least a part of therefrigerator; an upper part of the refrigerator to a surface of thecryostat around an opening of the vacuum sock; said vacuum sock having awall with a thermally conductive wall portion that in use, is thermallyand mechanically in contact with the second cooling stage of therefrigerator; thermal connectors that thermally connect the firstcooling stage of the refrigerator to a thermal radiation shield of thecryostat, said thermal connectors comprising a bellows structure that isin a retracted position when the vacuum sock is at atmospheric pressure,and that is in an extended position when the vacuum sock is under vacuumso the bellows presses a thermally conductive contact piece into thermaland mechanical contact with the first cooling stage of the refrigerator;and the refrigerator being elongate in an axial direction, and thebellows structure retracting and extending in a radial direction,perpendicular to the axial direction; and said second cooling stage ispressed in the axial direction in contact with the thermally conductiveportion of the wall of the vacuum sock, by atmospheric pressure actingon the upper part of the refrigerator, when the vacuum sock is undervacuum.
 2. An arrangement according to claim 1 comprising a thermallyconductive grease on mating surfaces of the contact piece and the firstcooling stage.
 3. An arrangement according to claim 1 comprising indiumon one or both of the mating surfaces of contact piece and the firstcooling stage. 4.-5. (canceled)
 6. An arrangement according to claim 1,wherein the bellows structure comprises an interface piece that isthermally linked to the thermal radiation shield by a thermal path whichthat includes a flexible thermal conductor
 7. An arrangement accordingto claim 6 wherein the flexible thermal conductor passes through thebellows structure.
 8. An arrangement according to claim 1 wherein thebellows structure is open at a radially outer end thereof, and aflexible thermal conductor passes from conductive contact piece, throughthe bellows structure and out of the bellows structure through the openradially outer end. 9.-12. (canceled)
 13. An arrangement for mounting atwo stage cryogenic refrigerator into a cryostat, the arrangementcomprising: a vacuum sock for accommodating at least a part of therefrigerator; attachment means for attaching an upper part of therefrigerator to a surface of the cryostat around an opening of thevacuum sock; a thermally conductive portion of a wall of the vacuum sockwhich, in use, is thermally and mechanically in contact with a secondcooling stage of the refrigerator; arrangements are provided forthermally connecting a first stage of the refrigerator to a thermalradiation shield of the cryostat; the said arrangements comprising abellows structure within the vacuum sock, the bellows structurecontaining a sealed mass of cryogen gas, whereby said bellows structureis in a retracted position when the vacuum sock is at atmosphericpressure, and which is in an extended position when the vacuum sock isunder vacuum, wherein, in its extended position, the bellows presses athermally conductive contact piece into thermal and mechanical contactwith a first cooling stage of the refrigerator; and wherein thethermally conductive contact piece is arranged to rotate about an axlein response to the bellows structure being in a retracted position whenthe vacuum sock is at atmospheric pressure, and being in an extendedposition when the vacuum sock is under vacuum.
 14. An arrangement asclaimed in claim 7, comprising a bearing surface, mechanically retainedin position with respect to said sock, and wherein said bellowsstructure is situated to bear on the contact piece and the bearingsurface.